Magnesium can be produced commercially by electrolysis of anhydrous magnesium chloride melts. It is well known that the drying process is the most complex and difficult stage in the electrolysis process in the production of magnesium. In fact, most of the R&D in magnesium production over the last decades focused on the drying process for production of anhydrous magnesium chloride with low content of magnesium oxide. The reason for this is to avoid three negative characteristics of the magnesium chloride hydrate hydrolysis reaction:                1. The creation of magnesium oxide, which will cause sludging in the electrolysis cells, which reacts with the graphite anodes and negatively impacts the energy efficiency of the electrolytic cell.        2. Losses of magnesium chloride during the process.        3. Some oxycompounds can participate in parasitic electrochemical reactions on the carbon anode consuming it in the process.        
Therefore, complete dehydration of MgCl2 must be conducted under conditions which minimize magnesium oxidation. Temperature and gaseous environment influence the formation of magnesium oxide compounds. To avoid that magnesium comes into contact with the air and so minimize the risk of oxidation, electrolysis cells operate under an inert atmosphere.
The dehydration of magnesium chloride hexahydrate is commonly carried out in a one or two stages drying process with hot air followed by one-stage of HCl drying. The first allow to remove between four and five hydrate content. Final dehydration of the MgCl2.2H2O must take place with large amount of dry HCl gas to prevent hydroxychloride formation (MgOHCl). This gas has to be dehydrated before recirculation.
By conventional electrolysis processes of magnesium metal production, dichlore gas (Cl2) is released then combined with hydrogen gas (H2) to form hydrogen chloride gas (HCl), which is used for drying magnesium chloride as indicated above. Presently, magnesium chloride feed preparation involves important costs, decreasing profits margins due mainly to the energy consumption to heat the gas at elevated temperature.
U.S. Pat. No. 3,760,050 describes a process for the preparation of substantially anhydrous magnesium chloride suitable for fusion electrolysis. The authors have put emphasis on size, mechanical strength and resistance to abrasion of magnesium chloride pellets. Accordingly, the manufacture of pellets involves balling together dehydrate prills and tetrahydrate molten in a rotating disk. The agglomerates are thereafter dehydrated in a shaft kiln by HCl gas at 330° C. for about 900 minutes. The disadvantage of this treatment is that it consumes a lot of energy to reach this temperature and maintain the heat condition for a long time. Furthermore, U.S. Pat. No. 3,760,050 does not disclose a mean to dehydrate hydrous HCl gas released in the described process for recycling in a continuous process of magnesium metal production.
Accordingly, there is thus still a need to be provided with a cost-effective process for producing magnesium metal from MgCl2.2H2O and recycling the HCl used during the dehydration process under an inert environment.